1. Field of the Invention
The present invention relates to a method and an apparatus for using a plurality of cells in a communication system supporting carrier aggregation between different evolved Node Bs (eNBs).
2. Description of the Related Art
Wireless communication systems have been developed that provide a high speed and high quality packet data service for communication standards such as, for example, High Speed Packet Access (HSPA) of 3rd Generation Partnership Project (3GPP), Long Term Evolution (LTE), High Rate Packet Data (HRPD) of 3GPP2, Ultra Mobile Broadband (UMB), and Institute of Electrical and Electronics Engineers (IEEE) 802.16e.
In an LTE system, a DownLink (DL) adopts an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an UpLink (UP) adopts a Single Carrier Frequency Division Multiple Access (SC-FDMA) scheme. The multiple access scheme helps distinguish data or control information for each user by providing time-frequency resources, which carry data or control information for each user, that do not overlap each other. Specifically, the time-frequency resources are allocated and operated to achieve orthogonality.
Further, the LTE system adopts a Hybrid Automatic Repeat request (HARQ) scheme in which a physical layer re-transmits corresponding data when a decoding failure occurs in an initial transmission. In the HARQ scheme, when a receiver does not accurately decode data, the receiver transmits information (Non-ACKnowledgement: NACK) informing a transmitter of a decoding failure. Thus, the transmitter may re-transmit the corresponding data through a physical layer. Further, the receiver may increase a data reception capability by combining existing data of which the decoding has failed and data re-transmitted by the transmitter. When the receiver accurately decodes data, the receiver transmits information (ACKnowledgement: ACK) informing the transmitter of a decoding success. Thus, the transmitter may transmit new data.
In providing a high speed wireless data server in a broadband wireless communication system, support of a scalable bandwidth is of significant importance. For example, the LTE system may support various bandwidths such as, for example, 20/15/10/5/3/1.4 MHz. Accordingly, service providers may select a particular bandwidth from the various bandwidths to provide a service. There are various kinds of terminals that may support a maximum bandwidth of 20 MHz and support only a minimum bandwidth of 1.4 MHz.
An LTE-Advanced (LTE-A) system, which aims to provide a service in a level of International Mobile Telecommunication (IMT)-Advanced demands, may provide a broadband service over a maximum bandwidth of 100 MHz through carrier aggregation between LTE carriers. For high speed data transmission, the LTE-A system requires wider bands than the LTE system, and terminals should receive a service through an access to the LTE-A system based on an importance of backward compatibility between the terminals. Accordingly, the LTE-A system divides an entire system band into subbands of the bandwidth, or Component Carriers (CCs), which can be transmitted or received by the terminal, and combines predetermined CCs. Further, the LTE-A system may generate and transmit data for each CC, and support high speed data transmission of the LTE-A through a transmission/reception process of the conventional LTE system used for each CC.
FIG. 1 is a diagram illustrating carrier aggregation in a general LTE-A system.
A configuration of FIG. 1 can be applied to uplink carrier aggregation as well as downlink carrier aggregation. An uplink refers to a wireless link in which a User Equipment (UE) 108 transmits data or a control signal to an eNB 102. A downlink refers to a wireless link in which the eNB 102 transmits data or a control signal to the UE 108.
Referring to FIG. 1, the eNB 102 supports aggregation of two CCs, CC#1 and CC#2. CC#1 includes a frequency f1, and CC#2 includes a frequency f2, which is different from the frequency f1. CC#1 and CC#2 may be operated by the same eNB 102, and the eNB 102 may provide coverages 104 and 106 corresponding to the component carriers. In the LTE-A system supporting the carrier aggregation, data transmission and control information transmission supporting the data transmission are basically performed for each CC.
Among the CCs included in the carrier aggregation, a CC that corresponds to a reference is called a primary carrier or a Primary Component Carrier (PCC), and a CC that is not the primary carrier is called a secondary carrier, a Secondary Component Carrier (SCC), or a non-primary CC. Information on a CC set and operated as the primary carrier, and information on the number of CCs to be aggregated are provided to the UE 108 by the eNB 102 through signaling.
In the downlink, a CC set as the primary carrier transmits initial system information or higher signaling, and may become a reference CC that controls mobility of the UE. In the uplink, a CC transmitting HARQ ACK/NACK for data received by the UE 108, or a control channel signal including a Channel Status indicator (CSI) indicating a status of a channel between the eNB 102 and the UE 108, may become the primary carrier. A cell including the downlink primary carrier and the uplink primary carrier is called a primary cell or a Pcell, and a cell including the downlink secondary carrier and the uplink secondary carrier is called a secondary cell or an Scell.
Further, in the carrier aggregation, there are two possible carrier aggregations, which include a symmetric carrier aggregation in which numbers of aggregated uplink CCs and downlink CCs are the same, and an asymmetric carrier aggregation in which numbers of aggregated uplink CCs and downlink CCs are different.
As described above, in the LTE-A system, the data is generated and transmitted for each CC, and scheduling information on the data transmitted for each CC is provided to the UE as Downlink Control Information (DCI). The DCI may be defined in various formats. Specifically, a DCI format is predetermined according to whether the DCI is an uplink grant (UL grant), which includes scheduling information on uplink data, or a downlink grant (DL grant), which includes scheduling information on downlink data, whether the DCI is a compact DCI having a small size of control information, whether the DCI applies spatial multiplexing using multiple antennas, and whether the DCI controls power. The predetermined format is applied and operated.
For example, DCI format 1, which is the DL grant including the scheduling information on the downlink data, includes the following control information.                Resource allocation type 0/1 flag: informs whether a resource allocation scheme corresponds to type 0 or type 1. Type 0 applies a bitmap scheme to allocate resources in units of Resource Block Groups (RBGs). A basic unit of the scheduling in the LTE and LTE-A systems is a Resource Block (RB) expressed by time and frequency domain resources. The RBG consists of a plurality of RBs and becomes a basic unit of scheduling in type 0. Type 1 allocates a particular RB within the RBG.        Resource block assignment: informs of RBs allocated for data transmission. Expressing resources are determined according to a system bandwidth and a resource allocation scheme.        Modulation and coding scheme: informs of a modulation scheme and a coding scheme used for data transmission.        HARQ process number: informs of a process number of HARQ.        New data indicator: informs whether HARQ transmission is an initial transmission or a re-transmission.        Redundancy version: informs of a redundancy version of HARQ.        TPC command for Physical Uplink Control Channel (PUCCH): informs of a Transmission Power Control (TPC) command for a PUCCH corresponding to an uplink control channel.        
The DCI is transmitted through a Physical Downlink Control Channel (PDCCH) corresponding to a downlink physical control channel via a channel coding and modulation process.
FIG. 2 is a diagram illustrating scheduling operations of the eNB and the UE according carrier aggregation in the general LET-A system.
Referring to FIG. 2, DCI 201 transmitted through CC#1 209 is channel-coded and interleaved after an application of a format defined in the LTE-A system, and then generates a PDCCH 203 signal. The PDCCH 203 signal includes scheduling information on a Physical Downlink Shared Channel (PDSCH) 213 corresponding to a data channel allocated to the UE 108 in CC#1 209 and transmitted to the UE 108. Further, DCI 205 transmitted through CC#2 211 is channel-coded and interleaved after an application of a format defined in the LTE system, and then generates a PDCCH 207 signal. The PDCCH 207 signal includes scheduling information on a PDSCH 215 corresponding to a data channel allocated to the UE 108 in CC#2 211 and transmitted to the UE 108.
In an LTE-A system, in order to reduce unnecessary power consumption of the UE 108, each CC or cell may be activated or deactivated. Specifically, when there is no data to be transmitted through a downlink or an uplink, the eNB 102 allows the UE 108 to deactivate the corresponding CC or cell, and thus a transmission/reception operation of the UE 108 in the corresponding CC or cell is limited. Accordingly, unnecessary power consumption is reduced. Further, when data to be transmitted to the corresponding CC or cell is generated, the corresponding CC or cell is quickly activated, so that a transmission delay can be minimized and a system efficiency can be increased.